The present invention is related to an apparatus for moving an object in an environment that is isolated from the ambient environment, and more particularly to a facility for vibrating a prototype tube for a nuclear power plant steam generator while the tube is being tested for corrosion under severe pressure, thermal, and chemical conditions in a test vessel.
A typical pressurized water reactor includes, inter alia, a reactor vessel which contains nuclear fuel, a coolant such as pressurized water which is heated by the nuclear fuel, and means for monitoring and controlling the nuclear reaction. The water heated in the reactor vessel is conveyed from and returned to the vessel by a reactor coolant system which includes one or more reactor coolant loops, each loop including a steam generator through which the water is pumped before it is returned to the reactor vessel for re-heating. The steam generator is basically a heat exchanger which transfers heat from the reactor coolant system to water from a secondary source that is isolated from the water of the reactor coolant system; the transferred heat boils the water from the secondary source, and the resulting steam is conveyed to a turbine to generate electricity.
A typical steam generator includes a vessel for receiving water from the secondary source and a large number of vertically arranged hair-pin tubes for conveying water from the reactor coolant system. Typically these tubes are made of Inconel (a nickel alloy) and have outer diameters of 7/8 inch (2.2 cm). The hair-pin tubes have hair-pin portions that are oriented toward the top of the steam generator and have elongated straight portions that are bundled together by support plates which are disposed parallel to one another at spaced apart positions. The support plates have openings for passage of the tubes and openings for passage of bubbles from the boiling secondary water.
Although the water from the reactor coolant system (that is, the water flowing within the tubes of the steam generator) is quite pure, contaminants may be present in the water from the secondary source. These contaminants may cause corrosion, a problem which is particularly troublesome at the openings where the tubes pass through the support plates. Corrosion products may accumulate in the gap or crevice between a tube and an opening, causing stress which may lead to cracking of the tube. Such cracks are to be scrupulously avoided because, inter alia, they might permit water from the reactor coolant system to mix with the secondary water and cause environmental pollution.
A conventional facility for testing a straight portion of a prototype steam generator tube includes an elongated vessel which is partially filled with water having the impurities of interest, such as chlorides, sulfates, iron oxide sludge, copper, substances of various pH values, etc. The vessel has a bottom closure member through which passes the tube to be tested, the axis of the tube lying along the axis of the vessel. Within the vessel, the tube under test is closed at its upper end, and a plate having an opening through which the tube passes is wired to the tube in order to simulate a support plate. A narrow inner tube is inserted into the tube under test, the upper end of the inner tube having an opening positioned near the closure of the tube under test. Outside the vessel, fittings are connected to the tube under test and to the inner tube. To conduct the test, pressurized water at approximately 600.degree. F. (315.degree. C.) is injected via one fitting, flows upward into the test vessel between the walls of the tube under test and the inner tube, and flows out the other fitting through the inner tube. This flow of hot water causes boiling within the vessel, raising the pressure in the vessel to around 900 pounds per square inch (6.2.times.10.sup.7 dynes/cm.sup.2) and raising the temperature to a correspondingly high value (typically around 500.degree. F., or 260.degree. C.). After the elapse of a suitable period of time, determined by the test protocol, the vessel is disassembled and the tube under test is checked.
While the conventional apparatus described above simulates a steam generator in that it permits a prototype tube to be tested at high temperature and pressure while heat flows through the walls of the tube, an important physical activity which occurs within a steam generator is ignored. Vibration is present in an actual steam generator due to vigorous boiling and turbulent fluid flow. Vibrations from around 15 to around 40 Hz have been observed within steam generators, and it is not entirely clear how such vibrations influence corrosion, particularly at the openings in the support plates.